Mendelian Genetics
Playing with Peas Gregor Mendel Why Peas? Austrian Monk who lived in the 1800’s researched heredity in pea plants Why Peas? Avaliable in many varieties Can control mating
Mendel’s Terminology Character Trait True-bred Hybridization heritable feature, i.e., fur color Trait variant for a character, i.e., brown True-bred all offspring of same variety Hybridization crossing of 2 different true-breds Generations P- parents F1 (first filial)- 1st generation of offspring F2 (second filial)- offspring of F1 generation
Patterns of Inheritance Purple x purple purple offspring White x white white offspring Purple plant x white plant all purple offspring Two F1 generation (purple plants) crossed3:1 ratio of purple to white
The Law of Segregation Alternative versions of genes (alleles) account for variations in inherited characteristics For each character, an organism inherits 2 alleles, one from each parent dominant allele- fully expressed in the organism’s appearance recessive allele- has no noticeable effect on the organism’s appearance when a dominant allele is present The alleles for each character segregate (separate) during gamete production (meiosis).
Alleles Somatic cells have 2 copies of each chromosome Could have 2 of the same or 2 different alleles If a dominant allele is present, it will be expressed Recessive alleles are only expressed when there are no dominant alleles Homozygous: pair of the same alleles Heterozygous: two different alleles for a gene
Genotype vs. Phenotype Phenotype an organism’s traits Genotype an organism’s genetic makeup
Punnett Squares Gametes on top and left sides Combine to predict possible offspring Monohybrid cross one contrasting trait 4 boxes Dihybrid cross two contrasting traits 16 boxes
Testcross When there is a dominant phenotype, cross with a homozygous recessive to determine genotype All dominant offspring homozygous dominant ½ dominant, ½ recessive heterozygous
The Law of Independent Assortment When there is more than one trait, they will sort independently of each other Traits are not tied together just because they came from the same parent In a dihybrid cross, every seed shape allele combines with every seed color allele.
Laws of Probability Multiply probabilities of allele segregation possibilities to get probability of each result Add probabilities of mutually exclusive events to get probability that one or the other will occur
Special Cases Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: M and N blood groups
Not all traits are simple to figure out… Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types Pleiotropy: genes with multiple phenotypic effects Ex: sickle-cell anemia
Multiple genes for one trait Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color Polygenic Inheritance: two or more genes affect the phenotypic character Ex: human skin pigmentation Quantitative characters (blending)
Nature vs. Nurture Some phenotypes are multifactorial Genetics and the environment both influence the phenotype Ex: Soil pH determines hydrangea color
Pedigree Analysis Can trace traits throughout a family tree Males = Females = O Lines between males and females= mating Children in order of birth from left to right
Genetic Disorders Recessive disorders: Dominant disorders: Cystic fibrosis Tay-Sachs Sickle-cell Anemia Dominant disorders: Huntington’s Polydactyly Achondroplasia Testing: •amniocentesis •chorionic villus sampling (CVS)